In seismic exploration, acoustic waves are generated at, or near, the surface of the earth and send downward into the earth. Reflections occur at the interfaces of subsurface strata whose acoustic impedances differ, and are subsequently detected by groups of geophones postioned at the earth's surface. Each geophone group (which can comprise a number of individual geophones) produces a composite electrical signal which drives indicating and recording equipment usually located in a recording truck, the equipment being electrically connected to the geophone group through a geophone cable or segment of a cable.
Whether the acquisition of the seismic data occurs in remote regions of the world or in more urban areas, performance standards relating to quantity as well as quality must be met.
As to the former, seismic crews for Chevron Oil Company, (Western Division), a subsidiary of Standard Oil Company, of California, habitually generate about two miles of conventional in-line, common depth point (CDP), 2400% stackable data per day per crew. Thus, any crew time that must be taken away from its active collection duties, as, e.g., in testing the geophone group or the geophones themselves for faults, must be kept to a minimum.
Also, it may be readily appreciated that the geophones of the geophysical data acquisition system must satisfy a very difficult set of quality requirements.
For example, new methods of processing field geophysical data are constantly being developed which attempt to extract more and more information contained in the acquired geophysical data. One such method which makes necessary the careful placement of the geophone and source arrays relative to each other is set forth and taught in U.S. Pat. No. 3,719,924 for "Anti-aliasing of Spatial Frequencies by Geophone and Source Placement," F. Muir et al, assigned to the assignee of the present application. In the above-mentioned patent, geophones in a group are placed in a selected configuration relative to each other (and a seismic source) whereby when their voltage output is combined, there is suppression of undesired interfering ("aliasing") energy. Assuming the geophone group included 36 geophones and each flyer included nine geophones each, all that is usually required is to connect the resulting four flyers in parallel in order to provide the desired suppression of the aliasing energy. Of course, the primary sensors of the data -- the geophones -- should also be uniformly sensitive in order to evenly detect waves which may have been significantly attenuated in their travel through many thousands of feet of earth.
A further requirement of geophones in field use is that they must be rugged to withstand rough handling, such as being dragged over rough terrain and being forcibly planted into the ground. Because even a single field truck may carry literally thousands of geophones, they also need to be physically small, but without sacrificing mechanical and electrical stability and required sensitivity to seismic energy.